The present invention relates to a method for forming an interlayer insulating film and, more particularly, to a method for forming an interlayer insulating film having a low dielectric constant, which is necessary for a highly-integrated semiconductor device. A progress in high integration regarding the semiconductor device in recent years has resulted in a narrower interval between wiring lines. As the narrowed interval between the wiring lines causes an increase in capacitance between the wiring lines, a request has been made for formation of an interlayer insulating film, which has a low dielectric constant.
With recent progresses in high integration of an LSI device, the wiring line has been micronized and multilayered. There has also been an increase in capacitance between the wiring lines. Such an increase in capacitance has caused a great reduction, in an operating speed. Thus, improvement in this regard has been strongly demanded. As one of improvement measures, a method for reducing capacitance between the wiring lines has been studied. This method uses an interlayer insulating film, which has a dielectric constant lower than that of SiO2 currently used for an interlayer insulating film.
Typical interlayer insulating films of low dielectric constants currently under study are {circumflex over (1)} an SiOF film, and {circumflex over (2)} an organic insulating film of a low dielectric constant. Description will now be made of these films.
{circle around (1)} SiOF Film
An SiOF film is formed by using source gas containing F and substituting Sixe2x80x94F bond for a portion of Sixe2x80x94O bond in SiO2. This SiOF film has a relative dielectric constant, which is monotonically reduced as concentration of F in the film increases.
For forming such SiOF films, several methods have been reported (see p.82 of monthly periodical xe2x80x9cSemiconductor Worldxe2x80x9d, February issue of 1996). Most promising among these methods is one for forming an SiOF film by using SiH4, O2, Ar and SiF4 as source gases, and by a high-density plasma enhanced CVD method (HDPCVD method). A relative dielectric constant of an SiOF film formed by this method is in a range of 3.1 to 4.0 (varies depending on F concentration in the film). This value is lower than a relative dielectric constant 4.0 of SiO2, which has conventionally been used for the interlayer insulating film.
{circle around (2)} Organic Insulating Film of Low Dielectric Constant
As an insulating film which has a lower dielectric constant (3.0 or lower) compared with the SiOF film, an organic insulating film of a low dielectric constant is now a focus of attention. Table 1 shows a few organic insulating films of low dielectric constants, which have been reported, and respective relative dielectric constants and thermal decomposition temperatures thereof.
However, the SiOF film is disadvantageous in that an increase in concentration of F in the film leads to a reduction in moisture absorption resistance. The reduced moisture absorption resistance poses a serious problem, because a transistor characteristic and adhesion of an upper barrier metal layer are affected.
Peeling-off easily occurs in the organic insulating film of a low dielectric constant, because of bad adhesion with a silicon wafer or the SiO2 film. Furthermore, the organic insulating film is disadvantageous in that heat resistivity is low since a thermal decomposition temperature is around 400xc2x0 C. The disadvantage of low heat resistivity poses a problem for annealing a wafer at a high temperature.
It is an object of the present invention to provide a method for forming an interlayer insulating film of a low dielectric constant, which has good moisture absorption resistance and heat resistivity. It is another object of the invention to provide a semiconductor device, which employs the above method.
In accordance with the method of the invention for forming an interlayer insulating film, first, porous SiO2 film is formed on an object to be formed. This porous SiO2 film is formed by using a Chemical Vapor Deposition method which employs source gases containing TEOS (tetraethoxy silane) and O3, where the concentration of the O3 is lower than that necessary for oxidizing the TEOS. Accordingly, many voids are formed in the film. In other words, porosity is provided for the SiO2 film formed in this manner.
Therefore, a dielectric constant of the porous SiO2 film is smaller than that of a usual SiO2 film having no porosity.
In addition, a SiO2 film is formed on the porous SiO2 film. This SiO2 film is formed by a Chemical Vapor Deposition method which employs source gases containing TEOS and O3 where the concentration of the O3 is sufficient for oxidizing the TEOS. Accordingly, the SiO2 film fore in this manner becomes a dense SiO2 film that contains no CH and OH radicals.
Therefore, since the SiO2 film formed on the porous SiO2 film is dense, incursion of moisture into the porous SiO2 film can be prevented, and an interlayer insulating film having good moisture resistance can be formed.
Furthermore, since these SiO2 films consist mainly of Si and O, these films are expected to show better heat resistivity compared to the organic insulating films of the prior art.
Secondly, in accordance with the method of the present invention for forming an interlayer insulating film, Cl (chlorine) plasma treatment is performed for the object to be formed. Accordingly, Cl (chlorine) atoms are left on some portions of the surface of the object to be formed. Subsequently, an porous SiO2 film is formed on the object to be formed by a Chemical Vapor Deposition method which contains TEOS and O3 as source gases. At this time, the growth of the SiO2 film is prevented on some portions of the surface on which the Cl (chlorine) atoms have been left. Accordingly, many voids are, formed in the SiO2 film. In other words, porosity is provided for this SiO2 film formed in this manner.
Therefore, a dielectric constant of the porous SiO2 film is smaller than that of a usual SiO2 film having no porosity.
Furthermore, since the porous SiO2 film consists mainly of Si and O, heat resistivity of the film is expected to show better heat resistively compared to the organic insulating films of the prior art.
Thirdly, in accordance with the method of the present invention for forming an interlayer insulating film, a first insulating film is formed on the porous SiO2 film, which has been formed on the object to be formed, the object having been subjected to the Cl (chlorine) plasma treatment. Then, after the first insulating film is etched to be planarized, a cover insulating film is formed thereon.
In other words, by the cover insulating film, incursion of moisture into the porous SiO2 film can be prevented. Therefore, it is possible to form an interlayer insulating film, which has a planarized surface and good moisture absorption resistance and heat resistivity.
Furthermore, the method for forming the foregoing porous SiO2 film can be applied to a damascene process. According to the damascene process, a Cu (copper) wiring layer having small electric resistance can be formed. By combining the Cu (copper) wiring layer with the foregoing porous SiO2 film, it is possible to provide a semiconductor device where a parasitic capacitance of a wiring line is small, and a data processing speed is fast.
Fourthly, in accordance with the method of the present invention for forming an interlayer insulating film, after formation of the foregoing porous SiO2 film, H (hydrogen) plasma treatment is performed. Accordingly, an Sixe2x80x94H bond is substituted for a dangling bond of Si in an Sixe2x80x94O bond in the surface of the void, and the surface of the void can be made stable.
Therefore, incursion of moisture from the surface of the void can be prevented, and it is possible to form an interlayer insulating film which has good moisture absorption resistance.